Add link to a list of open source design communities on Introduction page

[rabbitholiness]

In the "Getting started" section of the introduction page we recommend that people join online communities related to open source design. It would be nice if there would be a link to a small, curated list of such online communities, to get people started.

I also wonder if it would make sense to mention Penpot as an open source design tool at this point.

[GBKS]

Good idea. Let's gather a list here and then add it.